Parental Controls - before I buy - Can I block all traffic EXCEPT 1 or 2 sites?

I am considering buying a new router that supports Parental Controls.  BEFORE I buy, I would like to know if I can SUCCESSFULLY block all internet traffic, with 1 or 2 exceptions for educational purposes?  I am sick and tired of my kids watching stuff they shouldn't be.  I only want them to visit 1 or 2 sites and add more if they are reviewed by me.  Is this possible?  They have Chromebooks, so I can take away access to apps...I just need to limit the http/https traffic.

Thank you!